Near-field Polarization Research Articles

Polarization-independent tetramer metastructure with multi-Fano resonances governed by quasi-bound states in the continuum

We propose what we believe to be a novel metastructure consisting of silicon nanoblock tetramer clusters and investigated theoretically and experimentally how to enhance surface sensing capabilities with polarization-independent properties through bound states in the continuum (BICs). By introducing square defects, three quasi-BIC modes are excited at the wavelengths of 1013.29 nm, 1109.57 nm, and 1310.73 nm with a modulation depth up to 100% and a Q-factor of 16618, while achieving a sensitivity of 256 nm/RIU and a figure of merit of 2519.7 RIU−1. Utilizing the finite-difference method in the time domain (FDTD), we have analyzed the electromagnetic near-field distributions, polarization properties, and robustness corresponding to resonance dips at different wavelengths. Our results reveal that the distinct electromagnetic near-field distributions excited by the quasi-BIC modes are characterized by the strong field confinement within the silica nanoblock tetramer clusters. The modes are primarily governed by magnetic quadrupole (MQ) resonance and toroidal dipole (TD) moment. Additionally, the polarization-independent properties of the metastructure are demonstrated to ensure that they can effectively respond to the incident light regardless of its polarization state, while the robustness is investigated to show the stability and reliability by shifting square defects. Moreover, the transmission spectra of the metastructure is experimentally verified by immersing the sample into CaCl2 solutions in different concentrations. This work plays a pivotal role in creating further novel and high-performance optical metastructures while demonstrating the potential applications of quasi-BIC modes in precision sensing in the future.

Tailoring near-field-mediated photon electron interactions with light polarization

Inelastic interaction of free-electrons with optical near fields has recently attracted attention for manipulating and shaping free-electron wavepackets. Understanding the nature and the dependence of the inelastic cross section on the polarization of the optical near-field is important for both fundamental aspects and the development of new applications in quantum-sensitive measurements. Here, we investigate the effect of the polarization and the spatial profile of plasmonic near-field distributions on shaping free-electrons and controlling the energy transfer mechanisms, but also tailoring the electron recoil. We particularly show that polarization of the exciting light can be used as a control knop for disseminating the acceleration and deceleration path ways via the experienced electron recoil. We also demonstrate the possibility of tailoring the shape of the localized plasmons by incorporating specific arrangements of nanorods to enhance or hamper the transversal and longitudinal recoils of free-electrons. Our findings open up a route towards plasmonic near-fields-engineering for the coherent manipulation and control of slow electron beams for creating desired shapes of electron wavepackets.

Millimeter-Wave Slot-Based Cavity Antennas With Flexibly-Chosen Linear Polarization

Slot-based cavity antennas are hailed as promising candidates for millimeter-wave applications. Nevertheless, the linear-polarization (LP) angle of their broadside main beam is limited by the slots etched on the cavity’s top surface. In this work, an innovative technique is developed to significantly improve the selection flexibility of their LP inclination angle. It is attained by an integration of a single-layer, closely spaced C-shaped patch surface. A TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">710</sub> -mode slot-based cavity antenna is employed as the base configuration, which radiates a broadside beam with its LP along <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\phi = 90^{\circ }$ </tex-math></inline-formula> . To effectively predict and monitor the polarization conversion of the surface-integrated TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">710</sub> -mode cavity antenna, an analysis method using a unit cavity extracted from its original cavity antenna is presented. A subsequent surface-integrated system with the specified 45° LP was then simulated, fabricated, and measured. The measured results validate that a 45° LP state is achieved with an operating bandwidth from 33.3 to 36.5 GHz. Further investigation is conducted to flexibly choose the LP direction from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\phi = 15^{\circ }$ </tex-math></inline-formula> to 165°. Two more examples with the fabricated antenna prototypes successfully radiate the specified <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\phi = 15^{\circ }$ </tex-math></inline-formula> and 75° LP beams, respectively. This near-field polarization conversion surface can be generalized to cavities with different resonant modes.

Rotating Permanent Magnet Antenna Array Based on Near-Field Polarization Modulation

As a new type of low-frequency magnetic antenna, the rotating permanent magnet antenna (RPMA) has great application potential in underwater and underground communication. An RPMA array based on near-field polarization modulation is presented in this letter. The near-field polarization modulation is proposed because the RPMA can radiate circularly polarized magnetic fields in the direction of the rotation axis. Since the RPMA is intrinsically an electrically small antenna, it can only work in the near-field area where the field decays with the third power of the distance. The 1 × 2 array, which can enhance the field strength, is manufactured and tested. The measured results of the axial ratio and phase difference show that the proposed method is effective when the offset angle is not greater than 5.7°. A communication experiment is also carried out, and the results show that the RPMA array can achieve a transmission rate of 2 bps.

Near-Field DOA-Range and Polarization Estimation Based on Exact Propagation Model with COLD Arrays

Near-Field DOA-Range and Polarization Estimation Based on Exact Propagation Model with COLD Arrays

Field/valley plasmonic meta-resonances in WS2 -metallic nanoantenna systems: Coherent dynamics for molding plasmon fields and valley polarization

We theoretically study spin-valley quantum coherence dynamics in a hybrid system consisting of a monolayer of transition metal dichalcogenide (${\mathrm{WS}}_{2}$) and an Ag nanoantenna. For this we map the time evolution of the Bloch and Stokes vectors to investigate, respectively, the dynamics of valley excitons and the states of polarization of plasmon near fields. The results show the formation of two types of collective resonances in the time domain, i.e., valley and field plasmonic meta-resonances (VPMR and FPMR). VPMR is shown as a coherently time-delayed ultrafast rotation of the Block vector. FPMR, on the other hand, occurs as an abrupt change in the near-field polarization of the Ag nanoantenna. The ``time of occurrence'' (resonance time) of such meta-resonances can be tuned using the intensity and polarization of the laser field responsible for the excitation of the system. Our results show that FPMR can decorate near fields of the Ag nanoantenna at different locations with various types of coherent-field dynamics with nanoscale spatial resolution. VPMR, on the other hand, offers an avenue to control the spin-valley states of transition metal dichalcogenide monolayers using minuscule changes in the intensity and polarization of the incident light.

Adaptive tip-enhanced nano-spectroscopy

Tip-enhanced nano-spectroscopy, such as tip-enhanced photoluminescence (TEPL) and tip-enhanced Raman spectroscopy (TERS), generally suffers from inconsistent signal enhancement and difficulty in polarization-resolved measurement. To address this problem, we present adaptive tip-enhanced nano-spectroscopy optimizing the nano-optical vector-field at the tip apex. Specifically, we demonstrate dynamic wavefront shaping of the excitation field to effectively couple light to the tip and adaptively control for enhanced sensitivity and polarization-controlled TEPL and TERS. Employing a sequence feedback algorithm, we achieve ~4.4 × 104-fold TEPL enhancement of a WSe2 monolayer which is >2× larger than the normal TEPL intensity without wavefront shaping. In addition, with dynamical near-field polarization control in TERS, we demonstrate the investigation of conformational heterogeneity of brilliant cresyl blue molecules and the controllable observation of IR-active modes due to a large gradient field effect. Adaptive tip-enhanced nano-spectroscopy thus provides for a systematic approach towards computational nanoscopy making optical nano-imaging more robust and widely deployable.

Light–Matter Interaction Enhancement in Anisotropic 2D Black Phosphorus via Polarization-Tailoring Nano-Optics

Black phosphorus (bP), a two-dimensional (2D) layered material, has shown great potential for infrared (IR) optoelectronics owing to the narrow and direct bandgap it exhibits when in multilayer form. However, its thinness and optical anisotropy lead to weak light absorption, which limits the performance of bP-based photodetectors. In this work, we explore plasmonic nanoantennas, optical cavities, and their hybrids that can be integrated with multilayer bP to enhance its light absorption. This is achieved by near-field light intensity enhancement and polarization conversion. In addition, we demonstrate that these nanostructures can boost the spontaneous emission from bP. Light absorption enhancements of up to 185 and 16 times are obtained for linearly polarized and unpolarized IR light, respectively, in comparison with a commonly used device architecture (bP on SiO2/Si). Moreover, IR light emission enhancements of up to 18 times are achieved. The optical nanostructures presented here can be exploited for enhancing the detectivity of photodetectors and electroluminescence efficiency of light-emitting diodes based on bP and other 2D materials.

Development of Thermophysical Property Sensing Technique using Near-field Optical Polarization

Development of Thermophysical Property Sensing Technique using Near-field Optical Polarization

Tip-enhanced photoluminescence nano-spectroscopy and nano-imaging

Abstract Photoluminescence (PL), a photo-excited spontaneous emission process, provides a wealth of optical and electronic properties of materials, which enable microscopic and spectroscopic imaging, biomedical sensing and diagnosis, and a range of photonic device applications. However, conventional far-field PL measurements have limitations in sensitivity and spatial resolution, especially to investigate single nano-materials or nano-scale dimension of them. In contrast, tip-enhanced photoluminescence (TEPL) nano-spectroscopy provides an extremely high sensitivity with &lt;10 nm spatial resolution, which allows the desired nano-scale characterizations. With outstanding and unique optical properties, low-dimensional quantum materials have recently attracted much attention, and TEPL characterizations, i. e., probing and imaging, and even control at the nano-scale, have been extensively studied. In this review, we discuss the fundamental working mechanism of PL enhancement by plasmonic tip, and then highlight recent advances in TEPL studies for low-dimensional quantum materials. Finally, we discuss several remaining challenges of TEPL nano-spectroscopy and nano-imaging, such as implementation in non-ambient media and in situ environments, limitations in sample structure, and control of near-field polarization, with perspectives of the approach and its applications.

Near-field polarization of a high-refractive-index dielectric nanosphere on a dielectric substrate

The near-field polarization structure of a high-refractive-index dielectric nanosphere in a nonmagnetic and nonabsorbing medium is studied numerically in free space and in the vicinity of a flat dielectric substrate. Polarization patterns of the near field are obtained for linear and circular polarizations of the incident light and visualized using the polarization degree and three-dimensional Stokes parameters. Calculations are performed using the finite-element method and verified analytically using the Mie solution for scattering by a sphere with an extension of Weyl's method allowing description of reflection by a flat substrate. It is shown that the nanosphere drastically affects the polarization leading to a complex butterflylike structure. For instance, the near-field polarization is found to be qualitatively modified near the Mie electric and magnetic dipole and quadrupole resonances: linear polarization of the incident field results in circular polarization in some areas around the particle and vice versa. The areas of strong polarization change are found to be spatially localized in certain places around the particle making it possible to perform an experimental mapping. It is shown how a dielectric substrate affects the polarization distribution of the near-field of the particle. Also, several schemes of potential experiments to map the near-field intensity and polarization have been discussed.

Dual-band in situ molecular spectroscopy using single-sized Al-disk perfect absorbers.

We propose antenna-enhanced infrared vibrational spectroscopy by adopting single-sized Al disks on Al2O3-Al films fabricated by colloidal-mask lithography. The precisely designed plasmonic resonator with dual-band perfect absorption (DPA) shows strongly-enhanced nearfield intensity and polarization independence, at both resonances, providing a powerful antenna platform for the multi-band vibrational sensing. As a proof of concept, we experimentally apply the plasmonic DPAs in bond-selective dual-band infrared sensing of an ultrathin polydimethylsiloxane (PDMS) film, simultaneously amplifying two representative vibrational bands (asymmetric C-H stretching of CH3 at 2962 cm-1 and CH3 deformation of Si-CH3 at 1263 cm-1) by surface-enhanced infrared absorption spectroscopy (SEIRA). The plasmonic DPA was successfully adopted for the in situ monitoring of reaction kinetics, by recording the spectral changes in C-H stretching and Si-CH3 deformation modes of a 10 nm PDMS elastomer, which are selectively enhanced by the two antenna resonances, during its gelation process. Our systematic study of the SEIRA spectra has demonstrated mode splitting and a clear avoided-crossing in the dispersion curve as a function of resonance frequency of DPA, manifesting itself as a promising basis for future polaritonic devices utilizing the hybridization between the molecular vibrational states and the enhanced light field.

Near-field polarization singularities at a planar nonlinear metamaterial with strong frequency dispersion

We have investigated the spatial distribution of the electric field vector in a light wave interacting with a planar metamaterial with strong dispersive and nonlinear optical properties. Its basic element consists of two spatially separated silver strips. It is shown that polarization singularities of the light field (lines) appear in the vicinity of these strips. The shapes and the locations of the lines are determined by the frequency and intensity of elliptically polarized monochromatic light incident on the metamaterial.

Optical polarization response at gold nanosheet edges probed by scanning near-field optical microscopy**Project supported by the National Key Basic Research Program of China (Grant No. 2013CB934004) and the Fundamental Research Funds for the Central Universities, China (Grant No. YWF-13-D2-XX-14).

Optical properties of metallic edge-like structures known as knife-edges are a topic of interest and possess potential applications in enhanced Raman scattering, optical trapping, etc. In this work, we investigate the near-field optical polarization response at the edge of a triangular gold nanosheet, which is synthesized by a wet chemical method. A homemade scanning near-field optical microscope (SNOM) in collection mode is adopted, which is able to accurately locate its probe at the edge during experiments. An uncoated straight fiber probe is used in the SNOM, because it still preserves the property of light polarization though it has the depolarization to some extent. By comparing near-field intensities at the edge and glass substrate, detected in different polarization directions of incident light, the edge-induced depolarization is found, which is supported by the finite differential time domain (FDTD) simulated results. The depolarized phenomenon in the near-field is similar to that in the far-field.

The Effect of a Silicon Substrate on the Directivity Pattern of Scattered Radiation of a Gold Nanospheroid and on Near-Field Polarization

We studied the radiation-directivity pattern and the near-field polarization of a spheroidal metallic nanoparticle located over a silicon substrate by interaction with a linearly and circularly polarized field. It is shown that the directivity pattern of the spheroidal particle near the silicon substrate becomes strongly asymmetric and forward scattering is predominant compared with the symmetric diagram of a particle in free space. The change of the near-field polarization of the nanoparticle in presence of the substrate is studied for different wavelengths in the vicinity of the plasmonic resonance. The near-field polarization is described using the generalized Stokes parameters, which allow pictorial visualization of results.

Solitary Oxygen Dopant Emission from Carbon Nanotubes Modified by Dielectric Metasurfaces.

All-dielectric metasurfaces made from arrays of high index nanoresonators supporting strong magnetic dipole modes have emerged as a low-loss alternative to plasmonic metasurfaces. Here we use oxygen-doped single-walled carbon nanotubes (SWCNTs) as quantum emitters and couple them to silicon metasurfaces to study effects of the magnetic dipole modes of the constituent nanoresonators on the photoluminescence (PL) of individual SWCNTs. We find that when in resonance, the magnetic mode of the silicon nanoresonators can lead to a moderate average PL enhancement of 0.8-4.0 of the SWCNTs, accompanied by an average increase in the radiative decay rate by a factor of 1.5-3.0. More interestingly, single dopant polarization experiments show an anomalous photoluminescence polarization rotation by coupling individual SWCNTs to silicon nanoresonators. Numerical simulations indicate that this is caused by modification of near-field polarization distribution at certain areas in the proximity of the silicon nanoresonators at the excitation wavelength, thus presenting an approach to control emission polarization. These findings indicate silicon nanoresonators as potential building blocks of quantum photonic circuits capable of manipulating PL intensity and polarization of single photon sources.

Vectorial Near-Field Imaging of Silicon Heterostructure Cavities in Air-Slot Waveguides

Air-slot photonic nanocavities have many relevant advantages for hybrid integration of external light emitters/absorbers in Si-based optoelectronic devices. Still an experimental major drawback is the difficulty of performing near-field polarization mapping of the photonic modes localized in air-regions. Here, we demonstrate that near-field resonant transmission can achieve high-fidelity vectorial imaging of the electric-field distribution of a nanocavity mode in the challenging case of a silicon heterostructure photonic-crystal cavity realized in an air-slot waveguide where the mode is localized both in air and dielectric regions. We discern and control the in-plane independent polarization components of the electric field of the photonic mode by exploiting the Fano imaging technique. The use of metallic coated aperture near-field probes allows to map the two polarizations with a deep sub-wavelength imaging ( $\lambda $ /15) resolution.

Resonance fluorescence of a two-level quantum emitter near a plasmonic nanoparticle: role of the near-field polarization

It is demonstrated that the interaction of a two-level quantum emitter (atom, molecule, etc) with a plasmonic nanoparticle (prolate nanospheroid) in an external laser field features either an essential increase (up to a few orders of magnitude) or reduction (up to a few times) of the total decay rate of the emitter in specific areas around the nanoparticle in contrast to its decay rate in a vacuum. It is also shown that the resonance fluorescence spectrum of the emitter in close proximity to a plasmonic nanoparticle is very sensitive to both the location of the emitter around the nanoparticle and to polarization of the near-field, which depends in turn on the polarization of the incident laser field. This can be used in engineering potential quantum optics experiments with quantum emitters in the near-field, as well as for 3D nanoscopy of the near-field by registering the resonance fluorescence spectra of quantum emitters scattered in the vicinity of a plasmonic nanoparticle.

Quantum sensing using coherent control of near-field polarization of quantum dot-metallic nanoparticle molecules

We theoretically study the impact of quantum coherence on the states of polarization of the plasmonic fields of a quantum dot-metallic nanoparticle system. Via tracing Stokes parameters we predict that, depending on the refractive index of the environment, such a system can pass through different states of polarization with certain ellipticity and handedness. We demonstrate that this allows the nanoparticle system to act as a quantum sensor, wherein ultrasmall changes in the refractive index can lead to distinct changes in the time-dependent evolution of states of polarization (Stokes vector) of the plasmonic fields. Our numerical analysis also shows how these states can become strongly dependent on the intensity and frequency of the laser field responsible for the generation of quantum coherence. Possible applications for high resolution investigation of conformational dynamics and structures of biological molecules are discussed.

Radiation Enhanced Vivaldi Antenna With Double-Antipodal Structure

This letter presents a novel Vivaldi antenna that can be used to the microwave measurement because of the character of near-field linear polarization plane-like waves. Compared to previous double-slot Vivaldi antenna with the same dimension, the proposed antenna's gain, standing-wave ratio, and radiation characteristics are both improved greatly above 12 GHz by the inclusion of the double-antipodal structure, corrugated edges, and a feature called “director.” Furthermore, the double-antipodal structure introduces a coupled E-field; this reduces total cross-polarization level of the antenna. The proposed antenna is measured, and results show that it has a 4.7- to 20-GHz impedance bandwidth and maintains 20-dB copolarization-to-cross-polarization ratio below 10 GHz and 15 dB below 16 GHz, and its gain increases from 10.3 to 14.8 dBi without a significant decline at high frequencies. Symmetrical feed network also results in better symmetry of the beam, especially in H-plane.